This proposed project will be primarily concerned with describing the development of the atrioventricular (AV) node and bundle in the laboratory ferret heart. The ferret is an excellent animal model for cardiovascular research and one of the major reasons is that it has a cardiac conduction system which is morphologically very similar to the human conduction system. Three morphological techniques, (PAS, and acetylcholinesterase staining, transmission electron microscopy) which have been used on a variety of species, but which have never all been used on the same species, will be used to examine the morphogenesis of the AV node and bundle. The project will also investigate the possible developmental contributions of the atrioventricular and bulboventricular specialized ring tissue to the AV node and bundle, and more importantly whether specialized ring tissue is found in the developing ferret heart during the single heart tube stage. This research will 1.) extend our knowedge of the development of the AV node and bundle; 2.) provide anatomical substrates for some of the functional properties of the perinatal conduction system; and 3.) aid in our understanding of the localization of the cardiac conduction system in congenitally defective hearts.